add pdf epub options

[SaranjeetKaur]

No description provided.

[hturner]

I made a few changes:

• A local copy of the image of the edit icon was required to compile the PDF version. I also added some alternative text for the image: all images should have this for accessibility. This comes out as a caption in the PDF.
• Updated the GitHub Action in .github/deplot_bookdown.yml so the PDF and EPUB get rendered - otherwise there will be no PDF or EPUB to download!
• Removed bs4_book output format - this would be an alternative to the gitbook HTML version we are currently using. It may have some advantages, but this is a separate issue from adding PDF/EPUB downloads and would require to update screenshots etc if we wanted to move to that. So we should stick to gitbook here.
• Updated the README with notes on handling of images and previewing PDF/EPUB

I've tested the PDF and EPUB downloads locally (you need to render them separately as noted in the README).

@SaranjeetKaur Can you review my changes then merge this in so we can test the GitHub Action?

[SaranjeetKaur]

Updated a minor typo. The changes look good to me. I'll merge this PR now. Thank you!